This invention relates to an improved process for the production of urethanes (carbamic acid esters) by reacting organic nitro compounds with carbon monoxide and compounds containing at least one hydroxy group in the presence of a noble metal and/or a noble metal compound of the 8th Secondary Group of the Periodic Table of Elements and a particular co-catalyst specified below.
Industrially, organic isocyanates are generally produced by reacting the corresponding amines with phosgene. For some time, efforts have been made to find an industrially workable process for producing organic isocyanates in which there is no need to use phosgene. In one such synthesis, organic nitro compounds are reacted with carbon monoxide and organic hydroxyl compounds to form the corresponding urethanes. The urethanes are subsequently split into isocyanates and compounds containing hydroxyl groups. In this process the urethane intermediate may be modified before it is split. Thus, it is possible, for example, to react the phenyl urethane obtainable from nitrobenzene, carbon monoxide and ethanol with formaldehyde to form the bis-urethane of 4,4'-diisocyanatodiphenyl methane and subsequently to convert the thus-obtained intermediate into 4,4'-diisocyanatodiphenyl methane with elimination of the ethanol.
The splitting of urethanes into the corresponding isocyanates and compounds containing hydroxyl groups is described in German Offenlegungsschrift No. 2,421,503 and in the publications discussed therein.
The patent literature describes primarily two types of catalyst for the production of urethanes. For example, German Offenlegungsschrift Nos. 2,343,826; 2,614,101 and 2,623,694 describe the reaction of organic nitro compounds with carbon monoxide and alcohols to form urethanes in the presence of selenium or selenium compounds. Good yields of urethane are obtained by this process with both mononitro and dinitro compounds. However, the selenium compounds (particularly the organoselenium compounds formed as intermediates during the reaction and hydrogen selenide) are extremely toxic and have to be quantitatively removed during the process. This removal may be accomplished by chemical reaction. Such additional treatment increases the expense of the process.
Noble metals (particularly palladium) in the presence of Lewis acids are described as suitable catalysts in German Offenlegungsshcrift Nos. 1,568,044 and 2,603,574. Anhydrous iron (III) chloride is mentioned therein as a particularly effective Lewis acid. Although good yields of urethane, based on the nitro compound present, are obtained using these catalysts, the yields are unsatisfactory when based on the hydroxy compound used. For example, when ethanol is used as the hydroxy component, large amounts of diethyl ether are obtained due to the presence of the Lewis acid. Corrosion of the fine steel autoclaves used as reaction vessels is a significant disadvantage to use of these noble metal/Lewis acid catalysts. Although the degree of corrosion may be reduced to a certain extent by the addition of organic bases, such as pyridine (German Offenlegungsschrift No. 2,603,574), this reduction is not sufficient to permit long term operation. Another disadvantage of these catalyst systems is difficulty in recycling due to the instability of the Lewis acids in the presence of the hydroxy compounds used. Some of these disadvantages are minimized by the processes described in U.S. Pat. No. 4,219,661 and U.S. Pat. No. 4,262,130. However, these processes, in which iron oxychloride and iron oxides and/or iron oxide hydrates in combination with compounds containing chlorides are used as co-catalyst, are attended by the disadvantage that the practical need for high reaction velocities and minimal corrosion still cannot be fully satisfied by the catalysts systems described therein.